1. Field of the Invention
The present invention relates to a neutralization network for compensating for the effects of interelement coupling in an antenna array. More particularly, it relates to a network for coupling a portion of the signal received at each of the elements of an antenna array to all the other elements of the antenna array in proper amplitude and phase to compensate for the signal spatially coupled from each of the antenna elements excited by an incident signal to all the other elements of the antenna array.
2. Description of the Prior Art
In certain antenna array applications attempts have been made to achieve particular results by combining the signals induced in or radiated by the elements of an antenna array as if such signals originated from independent, coherent sources. For instance, two closely spaced pairs of antenna elements positioned orthogonally have been used as a direction finding antenna to determine the relative bearing of a distant source from the antenna array. Such an array is known in the art as an interferometer. Theoretically, the phase difference between the signals received by the opposite elements of one pair of the antenna array varies by sin .beta. and the phase difference between signals received by the opposite elements of the other pair of the antenna array varies by cos .beta., where .beta. is the relative bearing angle from the antenna array to the source. The relative bearing angle .beta. is then found from the relationship: EQU .beta.=tan.sup.-1 [sin .beta./cos .beta.].
The above relationship holds true only if there is no interaction between the elements of the antenna array. In actuality, each of the elements of the antenna array reflects a portion of the incident wave toward all the other elements of the antenna array so that the true signal at each of the elements of the antenna array is a composite of the signal directly received at that element together with the signals reflected to that element from all the other elements of the antenna array. As a result, the phase and amplitude of the signals at each of the elements of the antenna array differ from the phase and amplitude of the signals which would be received by each of the elements of the antenna array if those elements were isolated from one another. Consequently, the relative bearing angle .beta. determined by phase comparison of the signals received by the elements of the antenna array is in error and such error varies with changes in the relative bearing angle .beta..
U.S. Pat. No. 4,855,748, issued Aug. 8, 1989 to R. L. Brandao et al. for "TCAS Bearing Estimation Receiver Using A 4 Element Antenna" and assigned to the same assignee as the present invention, discloses a receiver system that employs a four element antenna array of the interferometer type designed for use in the Traffic Alert and Collision Avoidance System (TCAS). The receiver system determines the relative bearing angle .beta. between the protected aircraft upon which the receiver is mounted, to an intruding aircraft, by comparing the phase of the signals from the intruding aircraft received by the opposite elements of the four element antenna array. The relative bearing angle .beta. is then computed from the above-stated relationship.
The receiver system includes means for determining and compensating for: (a) differences in phase delay between the transmission lines connecting the antenna elements to the receiver input; (b) differences in phase delay between the four receiver channels preceding phase detection; and (c) errors caused by phase detector non-linearities. Thus, compensation is made for phase errors originating within the receiver system but the error in .beta. (computed) caused by interaction of the elements of the antenna array remains uncorrected.
It is the primary object of the present invention to provide means for compensating for the effects of interelement coupling in a closely spaced multielement antenna array.
It is another object of the present invention to provide a network for coupling a portion of the signal induced in each element of a multielement antenna array to all the other elements of the antenna array in proper amplitude and phase so as to neutralize that portion of the signal induced in each antenna array element by reflections of an incident wave from all the other elements of the antenna array.
It is a further object of the present invention to provide a means for reducing errors in the measurement of relative bearing angles in applications involving a direction finding antenna of the interferometer type.
These and other objects and advantages of the present invention will become evident as an understanding of the invention is gained from the following complete description thereof and the accompanying drawings.